Solid materials, such as copper or aluminum, generally conduct heat more efficiently than do gases such as air. The thermal conductivity of a material describes the materials ability to conduct heat. Pure copper has a thermal conductivity of 386 Watts/meter degree centigrade (W/m° C.) at zero degrees centigrade. Air has a thermal conductivity of 0.024 W/m° C. When two solid pieces of material are mated together by means of mechanical bonding, entrapped voids exist at the mating surface, which require heat to flow through a combination of smoothly contacting parts of the surface and across the entrapped voids. In the region of the voids, the heat transfer is affected by the thermal conductivity of the entrapped gas, or lack thereof, depending on the particular design of the mating surface. In the region of the contacting surfaces the heat transfer is governed more closely by the thermal conductivity of the solid materials.
FIG. 1 illustrates, at 100, an existing heat source/sink assembly where three materials are joined together. With reference to FIG. 1, a heat source 102 is mated with an interface material 106. The interface material 106 is mated to a heat sink 104 by means of mechanical bonding. The mechanical bonding results in entrapped voids and a resulting gap as indicated by 150. A heat flow 110 is in the direction from the heat source 102 to the heat sink 104 along a temperature gradient where a temperature profile decreases in the direction of the arrow. The interface material 106 has a thickness 108. The thermal resistance of a material is inversely proportional to the thermal conductivity. A material with a high conduction coefficient, such as copper described above has a low thermal resistance. A gas, such as air described above has a high thermal resistance. At the interface of two materials joined together by mechanical bonding, thermal contact resistance exists.
FIG. 2 is a plot 200 of thermal resistance 204 verses interface material thickness 202. A linear variation of thermal resistance 206 with material thickness 202 indicates a non-zero thermal resistance Rc 208 when the material thickness is zero at 210 (contact resistance). Existing heat transfer assemblies, such as the one shown in FIG. 1, exhibit the contact resistance shown in FIG. 2 at 208 and 210. Contact resistance has the undesirable effect of reducing the amount of heat transferred from the heat source 102 to the heat sink 104 in FIG. 1.
At a molecular level, the contacting surfaces are actually very far apart due to the mechanical bonding. FIG. 3 illustrates, at 150, an atomic level view of an existing heat source/surface material interface. A distribution of heat source surface molecules is shown at 304. Heat source surface molecules 304 are representative of the atomic scale existing on the surface 112 from FIG. 1. A distribution of interface surface molecules is shown at 302. The interface surface molecules 302 are representative of those molecules existing on the surface 114 in FIG. 1.
Valence shells 310, of the heat source surface molecules 304, are shown as concentric circles with their respective atomic nuclei. Similarly, valence shells 308, of the interface surface molecules 302, are shown as concentric circles with their respective atomic nuclei. In between the heat source surface and the interface material surface are voids 312 defined by a gap 306. The gap 306 prevents the valence shells 308 from contacting the valence shells 310. The transfer of heat is impeded by the gap 308 and the voids 312 since the thermally excited heat source surface molecules 304 are not in molecular contact with the interface material-surface molecules 302. The thermal contact resistance results from the voids and resulting gap between the materials.
Existing heat transfer assemblies rely on mechanical bonds at the mating surfaces of adjacent material. Referring again to FIG. 1, typically, the interface material 106 is softer than either the heat source 102 or the heat sink 104. Mechanical bonding is achieved when the softer interface material 106 is pressed into the irregularities in the surface 112 of the heat source 102 and the surface 118 of the heat sink 104. Over time, and through use, the mechanical bonds can weaken and break. The already high thermal resistance 206 (FIG. 2) between the components 102, 106, and 104 of the hest transfer assembly 100 increases. This increase in thermal resistance results in a higher operating temperature for heat source 102. Adverse effects on an attached electronic device (not shown) are realized and the life expectancy of the associated system is jeopardized.